brcmfmac: allow setting wlan MAC address using device tree

This allows firmware to provide MAC address using device tree. Like in
case there is no MAC burned in wlan NVRAM.

Signed-off-by: Pavel Löbl <pavel@loebl.cz>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220506044246.67146-1-pavel@loebl.cz